The molecules of air inside of an inflated balloon are very far apart and moving rapidly. They collide with other molecules and with the inside surface of the balloon. The volume of the balloon remains constant because the pressure caused by the air molecules colliding with the inside surface of the balloon equals the pressure caused by atmospheric molecules colliding with the outside surface of the balloon.
At normal temperature and pressure the air in a balloon behaves according to the Kinetic Molecular Theory of Gases, which states the following:
1. The molecules of a gas have insignificant volume compared to the distances between them. They are considered dimensionless points.
2. Gas molecules are in rapid, random motion. Collisions between gas molecules are perfectly elastic. Energy is transferred between molecules during collisions but no energy is lost.
3. There are no forces of attraction or repulsion between gas molecules.
4. The average kinetic energy of molecules in a sample of gas is proportional to the Kelvin temperature of the sample.
5. Gas particles move in a straight-line motion
If the temperature of the air in the balloon increases the molecules move faster and have more collisions. The volume of the balloon increases as the molecules collide more frequently with its inside surface. Conversely, when the air in the balloon is cooled the molecules slow down and the volume of the balloon decreases.
Chang, Raymond. Chemistry. 10th ed. New York: McGraw-Hill, 2010. Print.